1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image to a receiving material upon application of heat and pressure thereto, and to an image forming apparatus having the fixing device.
2. Discussion of the Related Art
Electrophotographic image forming apparatuses such as copiers and laser printers form an image by performing the following processes:
(1) forming an electrostatic latent image on an image bearing member;
(2) developing the electrostatic latent image using a developing device to form an unfixed toner image on the image bearing member;
(3) transferring the unfixed toner image onto a receiving material (such as paper sheets); and
(4) fixing the toner image to the receiving material using a fixing device.
A key component is the fixing device, and various fixing devices have been proposed. Specific examples thereof include heat roller fixing devices and belt fixing devices. In heat roller fixing devices, a receiving material bearing an unfixed toner image is passed through a fixing nip formed by a fixing roller having a heater therein and a pressing roller contacting the fixing roller, so that the unfixed toner image is fixed to the receiving material upon application of heat and pressure at the fixing nip. In belt fixing devices, a receiving material bearing an unfixed toner image is passed through a fixing nip formed by an endless fixing belt tightly stretched by a fixing roller and a heating roller while heated by the heating roller and pressed against a pressing roller contacting the fixing roller with the fixing belt therebetween, so that the unfixed toner image is fixed to the receiving material upon application of heat and pressure at the fixing nip.
Since there is an optimum temperature for fixing a toner image, the temperature of each of the fixing member (whether a roller or a belt) and the pressing member is typically controlled so as to fall in a temperature range including the optimum temperature by automatically providing or turning off electricity to heat sources for heating the fixing member and the pressing member. Such automatic control is achieved, for example, by a method including measuring the temperatures of a fixing member and a pressing member using a contact temperature detector (such as a thermistor) contacted with the fixing member and pressing member or a non-contact temperature detector (such as thermopiles using infrared rays), and sending the detected temperature information to a temperature controller to control power supply to heat sources for heating the fixing member and the pressing member.
However, even when the temperature is controlled as described above, the fixation property of a fixed image still varies somewhat depending on ambient temperature and humidity. Specifically, when the ambient temperature is low, the temperature of a receiving material declines, thereby degrading the fixation property of an image fixed on the receiving material. Similarly, when the ambient humidity is high, a recording paper absorbs a large amount of moisture, thereby degrading the fixation property of an image fixed on the receiving material.
In attempting to prevent deterioration of the fixation property, a fixing device is proposed which detects the ambient temperature and humidity using a detector to control the temperature of a fixing member based on the output from the detector. In addition, an image forming apparatus is proposed in which a temperature difference between a fixing member and a pressing member is controlled so as to fall in a predetermined range to maintain the fixation property.
However, heat transfer from the fixing member to the pressing member is not considered in the proposed fixing device and image forming apparatus. Therefore, the temperature of the pressing member cannot be controlled in the desired range, resulting in formation of a curled print.
For these reasons, the inventor recognized that there is a need for an image forming apparatus capable of producing prints having a good fixing property without curling.